#include <iostream>
#include <cmath>

using namespace std;

int main()
{
	double res=1, pi=0;
	double i=1, s=1;
	double x = 1.0/sqrt(3);
	while(1){
		res = pow(x, i)/i;
		if(res<=0.000001)
			break;
		pi += res*s;
		i+=2;
		s=-s;
	}
	printf("%.10f", 6*pi);
	return 0;
}
